1. Field of the Invention
This invention relates generally to thermal energy storage devices, and more particularly to thermal energy storage devices that store thermal energy in the form of latent and sensible heat.
2. Description of the Background Art
Thermal energy storage systems are a fast growing and evolving technology, with untapped commercial applications in the area of storage of energy from renewable (solar and wind) sources. For the most part, renewable energy generation systems are effective in delivering electrical power when the sun is shining or the wind is blowing, but not otherwise. For this reason, these systems are generally regarded as supplementary to conventional base-load generation systems (e.g., coal-fired power plants) and must operate on the grid so that power can be available from other sources at night or during cloudy or windless periods. Excess power being generated from renewable sources can be sold into the grid. However, as these systems proliferate, the cost to upgrade the power grid to accommodate such intermittent power sources could prove prohibitively costly. At present, the approach has been to incorporate into the grid rotating standby systems which are activated when the renewable source intermittency becomes an issue. Such an approach substantially increases the overall cost of increasing the fraction of renewable power in the supply system.
As a result, there is a shift in market focus to distributed, mostly autonomous systems that can meet the power requirements of smaller communities. If these requirements are to be met by renewables, there is a need for an efficient means to store excess energy collected during production hours and to make it available at other times, thus assuring a constant source of power.
There is also a need for efficient and compact storage devices in the field of power supplies for earth satellites. When solar powered, these satellites can experience a loss of power production once each orbit, when the satellite passes on the side of the earth opposite the sun. Storage devices which can provide power during these eclipses can be an essential component of the satellite's power supply system.
Among the presently available technologies that are used to store energy are batteries, flywheels, compressed air, sensible heat, and latent heat storage. The latter two technologies store energy in thermal form. They are particularly applicable when the energy production system directly produces thermal energy, as is the case with solar power and other heat-based systems.
In systems that convert thermal energy directly to electrical energy, it is important that thermal storage system be capable of operating at high temperatures that are matched to the operating temperatures of the thermal-to-electrical conversion devices such as, for example, Rankine, Brayton, or Stirling engines. Other desirable features of a thermal energy storage systems are compactness (i.e., high energy storage per unit volume), simplicity (minimal pumps or auxiliary heat exchange devices), and efficiency. Storage systems employing sensible heat fall short of many or all of these requirements.
Latent heat thermal storage systems overcome many of the above restrictions. In such systems, a material undergoes a phase change (from solid to liquid or from liquid to gas, and vice versa) as energy is stored or withdrawn. This material, commonly referred to as Phase Change Material (PCM), is chosen to be one in which the latent heat of fusion or the heat of vaporization per unit volume or per unit mass is large, thus enabling a large amount of energy to be stored using a small amount of material. The liquid and solid phases of the PCM co-exist and are in contact with each other. So long as both phases exist in equilibrium in the mixture, its temperature will be equal to the melt temperature and will remain constant. Salts, such as NaNO2, NaNO3, CaCl2, LiF, and KNO3, are typically considered as PCM candidates due to their high latent heat and appropriate melting temperatures.
The use of salt-based PCMs, which have low thermal conductivities, has presented a vexing problem for the direct extraction of stored energy. As heat is extracted, salt solidifies on the heat exchange surface and acts as an insulator, thereby impeding further transfer of heat from the liquid to the heat exchange surface. Because of the inherent properties of salts, thermal storage systems employing salt-based PCMs have relatively low heat extraction rates.
There have been many approaches to improving the heat extraction rates of thermal storage systems employing salt-based PCMs. For example, one approach includes partitioning the PCM-containing region using a material such as a ceramic or metal oxide which itself is capable of storing thermal energy as sensible heat. The partition material constrains the thickness of the solid PCM regions and acts as a conductor that provides a path for heat to be transported from the PCM to the heat exchange surface.
This concept has been extended to the use of a high conductivity matrix in which the PCM is embedded. This approach further limits the uninterrupted volume of the PCM and increases the area of contact between the PCM and heat transport material. One consequence of this approach is that the path for heat transport has an increased length. As another consequence to this approach, the effective volume of the PCM and, therefore, thermal energy storage capabilities of the storage device is decreased. Yet another consequence to this approach is that such systems are more sensitive to volume changes in the PCM during the melt/freeze process.
Others have suggested the use of scraping mechanisms to mechanically remove the solidified PCM from the heat exchange surface. Considering that a thermal energy storage unit is best if it is essentially a passive device (equivalent to a battery), the introduction of mechanical mechanisms with their attendant maintenance requirements is unattractive.
A fourth approach has been to use a secondary heat transfer fluid to convey heat from the PCM to the heat exchange surface. One instantiation employs floating an emulsified layer (produced by stirring) of metal in a salt PCM. Heat is transferred to the metal which is then used as a secondary heat transfer fluid. The liquid metal/liquid salt interface precludes development of an attached solid salt insulating layer. A clever variation of this approach was tested by Adinberg et al, who floated liquid sodium on a NaCl PCM. Solidification of the PCM occurs at the Na/NaCl interface from which the solid is removed naturally due to its negative buoyancy. Both these approaches are limited to only a select few PCMs and their associate temperature ranges. Furthermore, the use of sodium presents undesirable safety issues.
Note that all the previously suggested techniques entail undesirable restrictions on the working temperature range, mechanical devices, complexity, and/or reductions in the achievable energy storage density. In view of these limitations there is a need to develop thermal energy storage systems that escape the problem of insulating the heat exchange surface.